DVD manufacturing technology is relatively new and is rapidly evolving through a variety of different disc types, including two-sided discs.
A discussion of some recent attempts to produce DVDs appears in U.S. Pat. No. 6,180,200.
Adhesion of the DVD component disc layers, e.g. aluminum and polycarbonate disc layers has presented problems. For example, UV cure of adhesive requires line-of-sight exposure of the UV radiation, either directly on the adhesive or through various laminates.
Those laminates may consist of combinations of clear plastic and thin films of metal or thin films of metal coated onto the plastic. The UV radiation which reaches the adhesive through such a thin film of metal may be reduced to 0.1% of the original (free-radical (acrylate) processes sometimes use peroxide to overcome this problem), thus requiring a great excess of radiation be wasted, slowing the process, and further slowing total production line throughput, or requiring special procedures such as flash photolysis, or curing through the edge of the laminate.
Electron beam (EB) curable adhesives in combination with peroxide systems for DVD disc components are also known. With the appropriate EB processor accelerating voltage, discs can be cured through plastic, metal film, metal film coated on plastic, or a two-sided disc can be through-cured with one exposure. In contrast, it would be necessary for a UV-cured, two-sided disc of even the simplest design to be cured from both sides. EB processing allows the user to cure directly through all substrates (plastic, metal-coated plastic, paper, etc.) limited only by mass of substrate and accelerating voltage of the EB processor.
EB cationic cure is a simple process. Some UV systems, in order to achieve the requisite performance (adhesion, cure speed, etc.), require a hybrid process.
For example, U.S. Pat. No. 6,180,200 relates to such a hybrid method for bonding DVD disc layers through the use of a hybrid free radical and cationic pressure sensitive adhesive system to allow copolymerization of epoxy moieties with acrylate moieties. This hybrid system is said to have the cohesive strength of cationically cured resins combined with the fast cure and mild bonding characteristics of a pressure sensitive adhesive system (PSA). During cationic bonding, the discs are subjected to ultraviolet radiation. This system is said to have a faster cure rate than a cationic system alone. However the system is undesirably complex and difficult to control.
JP 11315132 discloses UV curable adhesives for DVDs, which adhesives contain glycidyl ether based epoxy resins and a photocationic polymerization initiator. This system has a chlorine concentration of 1 weight % or less. It suffers from the deficiencies of difficult process control associated with UV cure such as poor substrate penetration. In addition, the presence of chlorine may be hazardous and introduces an unnecessary degree of complexity to the manufacturing process.
JP 2058528 discloses a radiation curable composition for bonding optical disc components with an epoxy resin and a sulfonium salt. Organic peroxides and electron beam curing are employed. However, the organic peroxides tend to be unstable and process control is difficult.
It is an object of the present invention to overcome the above deficiencies of prior art DVD bonding methods.